Barack Obama was directly confronted on prime time television over allegations swirling around the internet that he is a secret Muslim who worships the Koran, during a Democratic presidential debate in Las Vegas.

Mr Obama, who was raised as a Christian by his white mother despite his Kenyan father being Muslim, has been dogged by smears and innuendo for months that he is in fact an Islamist trying to enter the Oval office by stealth.

"Let's make clear what the facts are: I am a Christian. I have been sworn in with a Bible. I pledge allegiance [to the American flag] and lead the pledge of allegiance sometimes in the United States Senate when I'm presiding."

Mr Obama is a member of Chicago's Trinity United Church of Christ and is an actively practising Christian. But opponents, particularly on the Republican side, delight in reminding people that his middle name is Hussein, taken from his father, a Harvard educated economist who left the family when Mr Obama was two.

After a deeply acrimonious dispute between Hillary Clinton and Mr Obama over the issue of race, which has dominated the campaign for over a week, the two blamed aides and campaign surrogates for the controversy and jointly pledged, on Martin Luther King's birthday, to put the matter behind them.

Yet both sought to gain the advantage on other issues. Mr Obama accused Mrs Clinton of taking a page from President Bush's playbook with earlier statements that the next commander-in-chief could expect to be tested quickly by terrorists - and that the next president would not have time to learn on the job.

Mr Obama said: "When Senator Clinton uses the spectre of a terrorist attack... I think that is part and parcel with what we've seen, the use of the fear of terrorism in scoring political points, and I think that's a mistake."

Asked during the NBC debate whether she had meant to say terrorists would test Mr Obama more than her, Mrs Clinton replied, "No, of course not," before adding, "it matters who's president."